1. Field of the Invention
The present invention relates to digital signal processing systems and, in particular, to digital variable symbol rate modulation for providing digital modulated signals over a continuous range of symbol rates.
2. Description of the Related Art
Digital data transmission from a transmitter to a receiver requires a variety of digital signal processing techniques to allow the data to be transmitted by the transmitter and successfully recovered by the receiver. In digital wireless telephone systems, a wireless telephone handset unit communicates via digital radio signals with a base unit, which is typically connected via a standard telephone line to an external telephone network. In this manner, a user m a y employ the wireless handset to engage in a telephone call with another user through the base unit and the telephone network.
Multi-line wireless telephone systems are in use in various situations, such as businesses with many telephone users. Such systems employ a handset that communicates with up to N handsets simultaneously, typically with digital communications schemes, such as a spread-spectrum, time division multiple access (TDMA). In a spread spectrum system, bandwidth resources are traded for performance gains, in accordance with the so-called Shannon theory. The advantages of a spread-spectrum system include low power spectral density, improved narrowband interference rejection, built-in selective addressing capability (with code selection), and inherent channel multiple access capability. Spread-spectrum systems employ a variety of techniques, including direct sequencing (DS), frequency hopping (FH), chirp systems, and hybrid DS/FH systems.
In a TDMA system, a single RF channel is used, and each handset transmits and receives audio data packets as well as non-audio data packets during dedicated time slices or slots within an overall TDMA cycle or epoch. Other communications schemes include frequency division multiple access (FDMA), code division multiple access (CDMA), and combinations of such schemes. Various modulation schemes are employed, such as carrierless amplitude/phase (CAP) and quadrature amplitude modulation (QAM).
Such digital data is often transmitted as modulated signals over a transmission medium, such as the RF channel, in the form of binary bits of data. (Other transmission media often used for digital communications include asymmetric digital subscriber loop (ADSL) systems or cable modem systems.) The digital data is often modulated and transmitted in complex digital data form, in which the transmitted data comprises symbols from which the original data can be reconstructed by the receiver. Complex digital symbol data typically comprises real (in-phase, or xe2x80x9cIxe2x80x9d) data, and imaginary (quadrature, or xe2x80x9cQxe2x80x9d) data (I, Q pairs). Each symbol of an I,Q pair may be a multi-bit number, and represent a point in a constellation on the I-Q plane. Each symbol is mapped or assigned to a prescribed coordinate in the I-Q plane, a four-quadrant Cartesian coordinate space using a look-up table (e.g., a ROM). The set of all symbol coordinates is the constellation. A prescribed number of symbols occupy assigned areas in the I-Q plane, depending on the modulation format. Depending on the number of bits/symbol of a given modulation format, the constellation contains a number of symbols at prescribed coordinates with respect to the I-Q axes. For example, in the QPSK modulation format, each sample has one of four(22) phase positions, one in each quadrant, so that each symbol pair represents two bits of data.
To transmit a given input data value in a complex data system, the input data value to be transmitted is mapped to a symbol pair or pair of coordinates I,Q of a corresponding constellation point on a complex signal constellation having real and imaginary axes I and Q. These I,Q symbols, which represent the original data value, are then transmitted as part of data packets by a modulated channel. A receiver can recover the I,Q pairs and determine the constellation location therefrom, and perform a reverse-mapping to provide the original input data value or a close approximation thereof.
In a spread spectrum system, each symbol is transmitted by a string of xe2x80x9csub-symbolsxe2x80x9d or xe2x80x9cchipsxe2x80x9d, derived by multiplying the symbol times a pseudo-random number (PN) binary string. Such systems are thus characterized by a chip rate, which is related to the symbol rate. Spread spectrum systems may also be used, in general, to transmit any digital data, whether in complex format or not.
As noted above, digital data transmission requires a variety of digital signal processing techniques to allow the data to be transmitted by the transmitter and successfully recovered by the receiver. For example, the receiver side of a data transmission in a spread-spectrum digital wireless telephone system employs a variety of functions to recover data from a transmitted RF signal. These functions can include: timing recovery for symbol synchronization, carrier recovery (frequency demodulation), and equalization. The receiver includes automatic gain control (AGC), carrier tracking loops (CTL), and equalizer loops for each link. Timing recovery is the process by which the receiver clock (timebase) is synchronized to the transmitter clock. This permits the received signal to be sampled at the optimum point in time to reduce the chance of a slicing error associated with decision-directed processing of received symbol values. In some receivers, the received signal is sampled at a multiple of the transmitter symbol rate. For example, some receivers sample the received signal at twice the transmitter symbol rate. In any event, the sampling clock of the receiver must be synchronized to the symbol clock of the transmitter.
Carrier recovery is the process by which a received RF signal, after being frequency shifted to a lower intermediate passband, is frequency shifted to baseband to permit recovery of the modulating baseband information. Equalization is a process which compensates for the effects of transmission channel disturbances upon the received signal. More specifically, equalization removes intersymbol interference (ISI) caused by transmission channel disturbances. ISI causes the value of a given symbol to be distorted by the values of preceding and following symbols. These and related functions, and related modulation schemes and systems, are discussed in greater detail in Edward A. Lee and David G. Messerschmitt, Digital Communication, 2d ed. (Boston: Kluwer Academic Publishers, 1994).
Receivers require a relatively stable source of a sampling clock signal, which is also controllable so that it can be locked to the transmitter symbol clock. Voltage controlled crystal oscillators (VCXOs) have been used for this function, since the clock signal produced by a VCXO is stable but controllable over a relatively narrow range, to allow it to be locked to the transmitter symbol clock. However, a VCXOs and similar components are analog and thus relatively expensive, and also prone to drift over time. In addition, if it is necessary to receive signals from different transmitters having different symbol clock frequencies, it is necessary to have a VCXO for each such transmitter, further increasing the cost of the receiver. An improved xe2x80x9cTiming recovery system for a digital signal processorxe2x80x9d is described in European Patent Application No. EP 0 793 363, European filing date Feb. 20, 1997, Applicant Thomson Consumer Electronics, Inc., inventors Knutson, Ramaswamy, and McNeely (Knutson et al.).
Another disadvantage to utilizing analog timebases in receivers is that, in a multiline wireless telephone system for example, since base and handsets employ different timebases, the base must track the timing offsets of many handsets in order to be able to acquire the signals for each handset during its time slot.
A wireless telephone system is presented having a plurality of wireless handsets and a base unit, with the base unit having a base transceiver. Each handset has a handset transceiver for establishing a wireless link over a shared channel with the base unit via the base transceiver, wherein the base transceiver transmits to a handset transceiver a first signal representing successive symbols at a first symbol rate. The handset transceiver has a receiver and a transmitter, and a local clock signal generator that provides clock signals at a local clock frequency. The receiver receives samples representing the first signal, and generates symbol error measurements used to cause a receiver interpolator to produce, in response to the received samples, samples taken at times synchronized to the successive symbols of the first signal. The handset transmitter transmits to the base transceiver a second signal representing successive symbols at a second symbol rate by modifying the second symbol rate in accordance with the symbol error measurements so that the second symbol rate is substantially identical to the first symbol rate.